1. Field of the Invention
The present invention relates to an optical pickup device for reading an information signal from and/or writing an information signal on an optical recording medium such as an optical disc, and an optical disc device which incorporates such an optical pickup device.
2. Description of the Prior Art
Heretofore, there have been proposed optical recording mediums such as optical discs and optical pickup devices for reading an information signal from such optical recording mediums. The optical pickup devices comprise a light source such as a semiconductor laser and optical components including an objective lens for converging a beam of light emitted by the light source onto a signal recording surface of the optical recording medium, and a beam splitter and other optical elements. The optical pickup devices read an information signal recorded on the signal recording surface by applying a converged beam of light onto the signal recording surface and detecting a beam of light reflected from the signal recording surface with a light detector.
An optical disc as an optical recording medium is held at its central area on a disc table in a disc player which incorporates an optical pickup device, and rotated about its own axis by a spindle motor coupled to the disc table. In the disc player, the optical pickup device is positioned so as to direct the objective lens toward the signal recording surface of the optical disc which is rotated. The optical pickup device is supported so as to be movable radially across the optical disc.
An information signal is recorded on the optical disc along a spiral pattern of recording tracks that are substantially concentrically arranged on the signal recording surface. When the optical disc is rotated about its own axis, the optical pickup device reads the recorded information signal along the recording tracks.
During rotation of the optical disc, the readout position where the optical pickup device reads an information signal from any of the recording tracks is periodically reciprocally moved radially of the optical disc because of an off-center deviation of the center of curvature of the recording track from the center of rotation of the optical disc, i.e., the central axis of the drive shaft of the spindle motor. In addition, the readout position on the signal recording surface is also periodically reciprocally moved perpendicularly to the signal recording surface due to a distortion or planarity error of the disc board of the optical disc.
To cope with such an off-center deviation and planarity error, the optical pickup device usually has an objective lens drive mechanism comprising a two-axis actuator. The objective lens drive mechanism supports the objective lens for movement along both an optical axis thereof and a direction normal to the optical axis, i.e., a radial direction of the optical disc. The objective lens drive mechanism moves the objective lens based on signals (a tracking error signal and a focusing error signal) which are representative of deviations or distances of the spot of light converged on the signal recording surface by the objective lens from the recording track in both the radial direction of the optical disc and the direction normal to the signal recording surface.
The beam of light emitted from the light source is guided to the objective lens by a light guide mechanism which comprises optical components including a beam splitter and is housed in a cavity defined in an optical unit housing along the optical axis of the objective lens.
The objective lens is moved by the objective lens drive mechanism to keep the beam of light focused on the recording tracks at all times so as to follow periodical fluctuations of the recording tracks.
As described above, the objective lens is moved by the objective lens drive mechanism in both the direction of the optical axis of the objective lens and the direction normal to the optical axis. The objective lens and the two-axis actuator of the objective lens drive mechanism are usually enclosed by a cover which prevents dust particles from entering and being deposited on the objective lens and the two-axis actuator. The cover has a through hole, whose size is greater than the diameter of the objective lens, defined in its portion in front of the objective lens for passing the beam of light therethrough, and hence the objective lens is exposed through the through hole. The cavity which houses the light guide mechanism is open and spaced from the objective lens by a predetermined distance. Consequently, while the disc player is in use or not in use, i.e., is in storage or shipment, dust particles tend to be deposited on the surface of the objective lens, and find their way into the cavity, possibly resulting in a failure to read the information signal from the optical disc. When dust particles are deposited on the two-axis actuator (which includes a support art, etc.), the objective lens drive mechanism may not be properly operated.
Particularly, fine sand particles or dust particles can easily be deposited on the objective lens and the support arm of the two-axis actuator, posing problems on maintaining desired playback characteristics with respect to the optical disc. The surface of the objective lens may be cleaned by a brush or an air blower. However, since fine sand particles or dust particles are liable to stick to the brush, they cannot sufficiently be removed from the surface of the objective lens with the brush. If fine sand particles or dust particles deposited on the surface of the objective lens are dampened, then it is difficult to remove them from the objective lens.
When dust particles and fine sand particles enter and are deposited in the cavity of the optical unit housing, it is highly difficult to remove them with the brush or the air blower.
In some applications, the disc player described above is incorporated into stationary unitary audio playback systems where the disc player is housed in a common casing shared by an amplifier, a radio tuner, etc. The casing necessarily requires an opening or openings for radiating the heat from the amplifier, the radio tuner, etc., and the optical components in the cavity, particularly the objective lens, cannot be cleaned with ease because the optical pickup device is housed in the casing. Stated otherwise, despite the fact that the entry of dust and dirt into the casing cannot be avoided, it is practically impossible to clean the optical components, particularly the objective lens, of the optical pickup device in the stationary unitary audio playback systems.